Of course he is going to be related to Connor. All of the protagonists are related to one another.
Although it makes more sense for Arno to be related to Jennifer Scott, rather than Haytham. After all, Haytham left England for America, but his half-sister's fate was largely unresolved. I wonder what it was that prompted Haytham to join the Templars, given that his father was an Assassin. I'm willing to bet that Haytham lost faith when Jennifer died, believing that the Templars could have saved her. As far as we know, Jennifer - and any family of her own that she had - stayed in England.
The setting reminds me of "The Scarlet Pimpernel" - a foppish, aristocratic young man leads a double life as the Scarlet Pimpernel, who saves the condemned during the French Revolution, because he believes in the power of the people, but thinks killing indiscriminately subverts the purpose of their uprising. He's basically a seventeenth-century Barman.